The present invention relates to an optical bidirectional transmission system, in particular, relates to such a system in which a center station or an optical subscriber unit communicates with a plurality of optical network units through optical fiber cables.
A PDS (Passive Double Star) system has been known as a bidirectional optical transmission system which operates on a time division basis between an optical subscriber unit and a plurality of optical network units.
FIG. 33 shows such a PDS system, in which the numeral 1 is a center station or an optical subscriber unit, 2-1 through 2-n are an optical network unit, and 3 is an optical star coupler which couples a transceiver in an optical subscriber unit 1 with a plurality of optical network units 2-1 through 2-n.
In a PDS system, a downward optical signal from an optical subscriber unit 1 to optical network units 2-1 through 2-n is multiplexed on a time division basis, and the multiplexed signal is split in the star coupler 3 into a plurality of optical signals, each of which is transmitted to a related optical network unit. In a receiving operation, each of the optical network units 2-1 through 2-n takes a time slot related to its own unit. In a transmitting operation, each of the optical network units transmits an optical signal in a designated time slot, and the star coupler 3 multiplexes all the signals of all the optical network units on a single optical line so that the optical subscriber unit 1 receives a time division multiplexed optical signal.
The PDS system has the advantage that a single tranceiver in an optical subscriber unit 1 may communicate with a plurality of optical network units, so that cost in an optical subscriber unit for each network unit may be low.
However, the PDS system has the disadvantage that an optical signal from an optical subscriber unit is attenuated in a star coupler depending upon a number of network units 2-1 through n. Therefore, the maximum number of network units coupled with a subscriber unit through a star coupler is restricted, and thus, the reduction of cost is also restricted.
FIG. 34 shows another prior art, in which an optical subscriber unit 1 has a laser oscillator 11, and an optical modulator which provides a time division multiplex signal. The star couple 3 in FIG. 33 is replaced by an optical switch 13 so that each time slot in a time division multiplexed signal is switched to a related optical network unit, instead of splitting the multiplexed signal.
Although the system in FIG. 34 has the advantage that a multiplexed optical signal is not attenuated in an optical switch, it has the disadvantage that the operation rate in a modulator must be high, and the signal control is complicated, depending upon a number of multiplexed network units.
Therefore, the reduction of cost in an optical subscriber unit for each network units is also restricted.